1. Field of the Invention
The present invention generally relates to a cooling fan and, more particularly, to a cooling fan able to prevent the top of a hub or a blade of a fan wheel from touching the bottom of a lid, such as a fan having an auxiliary shaft between the hub and the lid.
2. Description of the Related Art
Referring to FIG. 1, a conventional cooling fan 9 received inside a casing 90 to dissipate heat generated by a heat element 91 is shown. The cooling fan 9 has a base 92 where a shaft tube 93 is formed, with an outer periphery of the shaft tube 93 coupled with a stator 94, and with a bearing 95 received inside the shaft tube 93 and rotatably engaged with a shaft 961 of a fan wheel 96. The fan wheel 96 further includes a hub 962 and a plurality of blades 963 radially formed on the outer periphery of the hub 962. Top edges of the blades 963 are lower than or aligned with the top of the hub 962, and the fan wheel 96 also has a permanent magnet 964 to be driven by the stator 94. There is a gap “P” between the top of the hub 962 of the fan wheel 96 and a lid 97, so that the top of the hub 962 and the top edges of the blades 963 may not touch the lid 97 when the fan wheel 96 is rotated. Furthermore, a part of the shaft 961 of the fan wheel 96 can protrude from the bottom of the bearing 95, and a positioning ring can engage with the part of the shaft 961 protruding from the bottom of the bearing 95, to stabilize the axial position of the fan wheel 96 relative to the lid 97 when the fan wheel 96 is driven.
However, when there is only the gap “P” to prevent the top of the hub 962 and the top edges of the blades 963 from touching the lid 97, the fan wheel 96 easily goes upwards across the gap “P” and touches the lid 97, since the magnetic attraction between the permanent magnet 964 and the stator 94 is limited. Besides, although the positioning ring and the protruded part of the shaft 961 can further ensure the separation between the fan wheel 96 and the lid 97, the cooling fan 9 with these additional elements may lead to a complex structure that causes an increased manufacture cost. In light of this, it is desired to improve the conventional cooling fan.